(1) Field of the Invention
The present invention relates to a device and method for compensating for chromatic dispersion, and more particularly, to chromatic dispersion compensation device and method whereby the amount of dispersion can be set as desired.
(2) Description of the Related Art
In optical communication systems, signal degradation (chromatic dispersion) occurs as light pulses are propagated over a long distance through an optical fiber. Accordingly, such chromatic dispersion needs to be corrected to restore the optical signal to its original state.
Generally, a dispersion compensation fiber is used to correct chromatic dispersion. The dispersion compensation fiber has a fixed amount of dispersion, whereas the amount of chromatic dispersion of light pulses on a transmission path varies with time, depending on temperature change etc. With the dispersion compensation fiber, therefore, it is difficult to strictly compensate for the dispersion. Moreover, present-day large-capacity optical communication systems require extremely strict dispersion compensation techniques, and it is difficult to meet such requirements with the use of the dispersion compensation fiber.
In the circumstances, a chromatic dispersion compensation device of which the amount of dispersion can be set as desired has been devised. Such a dispersion compensation device will be hereinafter referred to as a VIPA (Virtually Imaged Phased Array) dispersion compensator.
FIG. 17 shows an example of a conventional VIPA dispersion compensator. The VIPA dispersion compensator comprises an optical circulator 911, an optical fiber 912, a collimating lens 913, a line focusing lens 914, a dispersion section 915, a line focusing lens 916, and a reflecting mirror section 917.
The dispersion section 915 includes a glass plate having a reflecting film with 100% reflectivity coated on an incidence side thereof except for the light incidence area and having a high-reflectivity reflecting film coated on a light emission side thereof (see Unexamined Japanese Patent Publication No. H09-43057, for example). The dispersion section 915 is slightly tilted with respect to the direction of incident light from the line focusing lens 914. The reflecting mirror section 917 comprises a mirror whose surface is curved in such a manner that concavity smoothly changes to convexity. In FIG. 17, the far side of the mirror constitutes a concave mirror, and the near side of same constitutes a convex mirror.
Light incident on the optical fiber 912 from the optical circulator 911 is propagated through the optical fiber 912 and then is turned into a parallel beam by the collimating lens 913. Subsequently, the parallel beam is focused by the line focusing lens 914 to be incident on the dispersion section 915.
The incident light undergoes multiple reflection within the dispersion section 915 and emerges therefrom. The emerging light behaves in the same manner as light emitted from a diffraction grating and therefore acts as diffracted light. This diffraction grating is not a real one but a virtual diffraction grating and is hence called a virtually imaged phased array (VIPA).
The diffracted light emitted in this manner has various orders of diffraction, and the following description is directed only to required orders (hereinafter “diffracted light”). The diffracted light is focused on the reflecting mirror section 917 by the line focusing lens 916. At this time, light waves of different wavelengths are focused at different locations on the reflecting mirror section 917. The focused light waves are reflected in various directions by the reflecting mirror section 917, then pass through the line focusing lens 916 and again reach the dispersion section 915. At this time, the light waves of different wavelengths arrive at different locations on the dispersion section 915. Consequently, the light waves require different periods of time to again reach the window of the light incidence side after undergoing multiple reflection within the dispersion section 915, thus producing group delay time.
By moving the reflecting mirror section 917 in the X-axis direction, it is possible to adjust the incidence positions of the reflected light on the dispersion section 915, namely, to adjust the group delay time (see Unexamined Japanese Patent Publication No. 2003-15076, and “Design for VIPA variable dispersion compensator simulator” by Hirotomo Izumi, Yasuhiro Yamauchi, and Yuichi Kawabata, Journal C of The Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 10, pp. 898-905, October 2002, for example).
In the conventional dispersion compensator, however, a single dispersion section functions as both a dispersion element and a delay element, and accordingly, optical adjustment is made taking account of the balance of the dispersion function and the delay function. This means that two parameters are adjusted by means of a single parameter, and thus there is no guarantee that both of the characteristics can always be optimized. Namely, if the optical system is adjusted so as to obtain a desired dispersion function, then the delay function may possibly fail to attain a desired value. Similarly, if the optical system is adjusted so as to obtain a desired delay function, the dispersion function may possibly fail to attain a desired value. Thus, in the conventional dispersion compensator, the dispersion function and the delay function are performed by a common optical system, and this makes it difficult to attain an optimum dispersion compensation function.